Ok, so if this were a completely free service I would agree that asking the mods to do all that work would be crazy, but as this is a game that is asking for money, you HAVE to do it. It's no longer in the realm of a backyard gaming experience when you start to charge for it (yes I realize that we can play for free as well, but if I choose to pay for the experience and this is what I get, an easily hackable game, then I'm not pleased and will take my business elsewhere).
So...I would suggest that NO ONE from the same IP address be allowed to play in the same game. This just makes sense to me on so many levels. If you play with your brother on the same IP address (this is the example often given by Joshua not only on this thread but others) then it is patently UNFAIR to play in the same game and will RUIN THE EXPERIENCE FOR EVERYONE ELSE.
It's nearly impossible to beat two people playing together (ok I exaggerate, but it is nearly so, especially on conquer where you can start right next to the other person and just never attack them, which I have seen NUMEROUS times), which is why I ALWAYS stipulate at the beginning of a game that there are to be no alliances.
I have other ideas about the alliance thing and how you can make it fair, but that's for another topic.
Obviously there is nothing, in this particular case that I'm suggesting, that would stop people from using proxy servers or playing from another IP address, i.e. friends who play together and split the wins.
You could make an easy program that flags people who play together often, which would give you a good starting place for further inquiry.
To be honest, I think that people who cheat in this way should be banned as well as their IP address on the first offense. This would severely limit the number of people doing it (though obviously it wouldn't stop it, it would make the level a little more acceptable, as it seems to be a small epidemic at the moment).
My two cents for the moment.